One or more embodiments of the present invention relate to methods and apparatus for performing optical scans. In particular, one or more embodiments of the present invention relate to method and apparatus for performing an optical coherence tomography (xe2x80x9cOCTxe2x80x9d) optical scan to image, for example, and without limitation, tissue and anatomical features of an eye.
An optical coherence tomography (xe2x80x9cOCTxe2x80x9d) apparatus (for example, like one disclosed in U.S. Pat. No. 5,321,501) is an optical imaging apparatus that can perform micron-resolution, cross-sectional imaging of biological tissue. As is well known, the quality of an OCT image depends on: (a) the resolution of the image (which resolution is related to the coherence length of a radiation source used in the OCT apparatus); and (b) the signal-to-noise ratio of the OCT image.
To increase the signal-to-noise ratio of the OCT image, scanning optics is optimized for a particular application so that a maximum amount of reflected or scattered OCT radiation can be collected by the scanning optics. Such optimization is important for scanning anterior segments of features of an eye such as, for example, an anterior surface of the cornea, because the curvature of, for example, the anterior corneal surface is steep, and much of the reflected OCT radiation is therefore directed away from an optical axis along which the reflected OCT radiation is collected.
A book entitled xe2x80x9cOptical Coherence Tomography of Ocular Diseasesxe2x80x9d by C. A. Puliafito et al, published by SLACK Incorporated, 1996, at p. 14 discloses scanning optics for use in imaging anterior segments of features of the eye. As shown in FIGS. 1-12 thereof, the scanning optics is a telecentric optical system that causes a beam of scanning OCT radiation to be parallel at various positions across the front of the eye. The disclosed scanning optics is problematic in such applications because the curvature of, for example, the anterior corneal surface is steep, and as a result, much of the reflected OCT radiation is lost. Consequently, a detected signal produced by such reflected OCT radiation is weak outside a small central section of the corneal surface.
This issue is addressed in U.S. Pat. No. 5,493,109 to J. Wei et al. which discloses scanning optics that causes chief rays of the scanning OCT radiation to be focused at a center of the cornea. As a result, the scanning OCT radiation is directed to impinge substantially perpendicular to the anterior corneal surface, and as a result, a maximum reflected signal is acquired.
However, the scanning optics disclosed in both of the above-referenced prior art references is problematic in that it traces out a scanning image over an inward curved surface, i.e., a surface whose curvature is opposite that of the anterior corneal surface. In particular, a point of best focus of the scanning beam (as it is scanned over the anterior corneal surface) lies on a curved surface whose curvature is opposite to the curvature of the anterior corneal surface.
In addition to the above, using prior art OCT apparatus, OCT longitudinal scans into the eye over depths larger than the thickness of the cornea are problematic. This is because the beam of scanning OCT radiation may be out of focus when the longitudinal scan depth extends deep into the eye. Hence, the detected signal strength will not be the same when the beam of scanning OCT radiation is scanned at different longitudinal depths into the eye. To overcome this problem, the width of the beam of scanning OCT radiation is designed to be small enough that the depth of focus of the beam is comparable to the longitudinal scan depth range required for a particular application. However, this causes a problem because reducing the width of the beam of scanning OCT radiation is equivalent to reducing the numerical aperture (xe2x80x9cN.A.xe2x80x9d) of the scanning optics. This, in turn, reduces the radiation collection efficiency of the scanning optics for reflected or scattered OCT radiation. For example, as is well known, radiation collection efficiency is proportional to the 2nd power of N.A. of the scanning optics, and depth of focus is inversely proportional to the 2nd power of N.A. As a result, the radiation collection efficiency of the scanning optics for reflected or scattered OCT radiation is proportional to the inverse of the longitudinal scan depth range. In addition, the problem is even worse when the point of best focus of the beam of scanning OCT radiation traces out a curved surface that is opposite to a desired scanning beam image geometry (i.e., for example, that of the geometry of the anterior corneal surface).
In light of the above, there is a need in the art for method and apparatus to solve one or more of the above-identified problems.
One or more embodiments of the present invention advantageously satisfy one or more of the above-identified problems. Specifically, one embodiment of the present invention is a scanner for a beam of scanning optical coherence tomography (xe2x80x9cOCTxe2x80x9d) radiation that comprises: (a) a source of OCT radiation; (b) a scanner; and (c) scanning optics whose image surface has a negative field curvature.